This invention relates in general to a control circuit for a light projection system, and more particularly to a control circuit wherein a voltage comparator compares the potential on one of a pair of input lines to a voltage reference generated by the circuit such that the circuit is capable of detecting failure of a main lamp and automatically energizing a spare lamp and is further capable of distinguishing between failure of the main lamp and operation of the projection system at reduced input voltages.
Various types electrically, mechanically and thermally activated devices are known in the prior art for energizing a spare lamp in a light projection system upon failure of the main lamp. Many of the mechanically and thermally activated devices are unsuitable for commercially used light projection systems because of the length of time taken between failure of the main lamp and energization of the spare lamp. Especially in commercial usage of light projection systems, such as presentations before large or important audiences including sales or marketing conferences, training sessions, conventions and the like, rapid sensing of the failure of the main lamp and energization of the spare lamp are required to avoid annoyance and inconvenience to everyone involved.
Prior art controls, including circuits, also suffer from the inability to distinguish between failure of the main lamp and operation of the main lamp at reduced voltages. In those control systems which employ circuitry, the circuitry is generally operated from direct current voltage sources, requiring expensive and inefficient means to isolate the alternating current voltage source generally associated with projection systems from direct current voltage source required for the circuitry. Motors, transformers, photoelectric devices and thermal sensors are typical of these isolation techniques.